1. Field of the Invention
This invention relates to a diamond-coated hard material having a diamond-or diamond-like carbon-coating layer with a high bonding strength to a substrate.
2. Description of the Prior Art
Diamond having many excellent properties, for example, very high hardness, chemical stability, high heat conductivity, high sound wave propagation speed, etc. has widely been used as hard materials utilizing these properties or diamond or diamond-like carbon coated hard materials, illustrative of which are as follows:
1 single crystal diamond, sintered diamonds or diamond-coated cutting tools such as throwaway inserts, drills, microdrills, endmills, etc., which are capable of cutting Al, Cu, various practically used light metals or alloys thereof at a high temperature and obtaining well finished surfaces, because of hardly reacting with these metals or alloys.
2 various wear resistance tools such as bonding tools capable of working for a long time with a high dimensional precision, because of high wear resistance.
3 various machine parts such as radiating plates.
4 various vibration plates such as speakers.
5 various electronic parts.
In the production of artificial diamond, there are methods of forming diamond coating layers from gaseous phase, for example, microwave plasma CVD method, RF-plasma CVD method, EA-CVD method, induction field microwave plasma CVD method, RF hot plasma CVD method, DC plasma CVD method, DC plasma jet method, filament hot CVD method, combustion method and like. These methods are useful for the production of diamond-coated hard materials.
Since many of the diamond-coated hard materials are lacking in bonding strength of the diamond-coated layer to a substrate, the diamond-coated layer is stripped to shorten the life in many cases. The great reason therefor is that diamond not capable of forming intermediate layers with all materials has low wettability with other materials.
In order to obtain a diamond-coated hard material having a high bonding strength, there have been proposed a method comprising selecting a substrate having the same heat expansion coefficient as diamond (Japanese Patent laid-Open Publication No. 291493/1986, proposing sintered compacts containing Si.sub.3 N.sub.4 as a predominant component, and sintered compacts containing SiC as a predominant component), a method comprising subjecting the surface of a substrate to etching to remove metals having bad infueneces upon formation of the diamond coating layer on the substrate surface and thereby increasing the formation density of diamond nuclei on the substrate surface (Japanese Patent Laid-Open Publication No. 201475/1989, etching the surface of a cemented carbide with an acid solution to remove Co metal component and to suppress graphitization of the diamond nuclei; Japanese Patent Laid-Open Publication No. 124573/1986, subjecting the surface of a substrate to a scratching treatment with diamond grains or a diamond wheel and thereby improving the nuclei forming density on the surface of the substrate), etc.
As described above, diamond is chemically stable and does not form intermediate compounds with all materials. When a diamond-coated hard material having an excellent bonding strength is prepared, therefore, such a condition must be provided that a diamond coating layer and a substrate are bonded by a strong physical strength.
The inventors have made studies to realize this condition and consequently, have found that when protrusions having a high bonding strength to a substrate are formed on the surface of the substrate by a chemical or mechanical means and a diamond coating layer is formed thereon, thereby forming such a state that protrusions are intruded into the diamond coating layer, the bonding strength between the diamond coating layer and the substrate is rendered very high. This can be considered to be due to that the contact area of the diamond coating layer with the substrate is increased and the protrusions have anchor action in the diamond coating layer, whereby the diamond coating layer is hard to be stripped from the substrate.